Neutralization of ion plasma oscillations in electron discharge tubes



1957 K. G.'HERNQVIST I NEUTRALIZATIQN 0F ION PLASMA OSCILLATIONS mELECTRON DISCHARGE TUBES Filed Feb. 1954 SQURCE BY ATTORNEY t n edStates. Patent 9 NEUTRALEZATION FF ION PLASMA OSCILLA- TION S INELEQTRQN DISCHARGE TUBES Karl G. Hernqvist, Princeton, N. 5., assign-orto Radio Corporation of America, a corporation of Delaware ApplicationFebruary 1, 1954, Serial No. 407,287 8 Claims. (Cl. 315-538) Thisinvention relates to neutralization of ion plasma oscillations, andparticularly to means for generating a uniform unmodulated high densitybeam of charged particles such as an electron beam.

It is known to generate a conical electron beam of high density at theapex by thermally emitting electrons from a concave cathode, thru aconcentric accelerating grid, and thru an aperture in an anode to a beamutilizing device. The beam utilizing device, for example, may be aklystron, a magnetron, a traveling Wave tube or an impedancetransformer.

The electrons in the beam repel each other and resist being concentratedinto a dense beam at the apex. To make a very dense beam, the mutualrepulsion of the electrons must be overcome. This may be done byenclosing the beam in a region substantially free of an electric fieldand having a potential greater than the ionization potential of theresidual gas. This potential may be in the order of to volts. Positiveions are formed by collisions of the electrons with residual gas atoms,and since the ions are in a field-free region, they remain to form apositive ion plasma which neutralizes the mutual repulsion of theelectrons in the beam. This is known as space charge neutralization.

It has been found that in a space charge neutralized beam, strayelectrons strike an electrode in contact with the ion plasma, and thruthe effects of secondary emission, the ion plasma is caused to oscillateat a frequency in the order of 0.5 to 10 megacycles depending on the iondensity. The ion plasma oscillates at a frequency which varies directlyas the square root of the ion density. Under the equilibrium conditionsexisting, the ion density is equal to the electron density. Thereforethe ion plasma oscillates at a frequency proportional to the square rootof the electron density. The ion plasma oscillates at a naturalfrequency according to the formula:

f= 2.15k1/ L Mw V where f is frequency in megacycles, k is a geometryfactor which is equal to the number 1 for plane parallel oscillations ofthe plasma and is equal to l/ /2 for a cylindrical plasma column, Ia. iselectron density in terms of milliamperes of anode current per squarecentimeter of efifective anode area, Va is anode voltage and M is themolecular weight of the residual gas molecules. The number 2.15 is aconstant including such factors as electron charge, electron mass,dielectric constant, etc.

The ion plasma oscillations tend to cause a modulation of the electronbeam passing thru the aperture in the anode to the utilization device.It is therefore a general object of this invention to provide means forneutralizing the undesired modulation of a beam of charged particlessuch as an electron beam caused by the ion plasma oscillations.

It is another object to provide novel means to generate an unmodulatedelectron beam of high density.

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In one aspect, the invention comprises a vacuum tube having a concavecathode, an accelerating grid concentric therewith, and an aperturedanode constructed to cooperate with the grid to form a field-freeregion. A positive ion plasma is formed in the field-free region toprovide space charge neutralization which allows the electron beam to behighly concentrated in passing thru the aperture in the anode. Anundesired modulation of the electron beam passing thru the aperture inthe anode results from oscillations of the ion plasma. This modulationof the electron beam is neutralized by taking a signal from the anode,passing it thru an amplifier and a phase adjuster, and applying thesignal in proper phase to the accelerating grid. In this way, themodulation of the electron beam where it passes thru the aperture in theanode to the utilization device is substantially eliminated.

These and other objects and aspects of the invention will be apparent tothose skilled in the art from the following more detailed description ofthe invention taken in conjunction with the appended drawing, the singlefigure thereof showing one embodiment of the invention.

The drawing shows a vacuum tube together with feedback means toneutralize modulation of the electron beam caused by ion plasmaoscillations. The cathode 5 is in concave form (a segment of a sphere),and is heated by heater Wire 6 connected to a source of heater current7. An accelerating grid 8 is arranged concentrically with but spacedfrom the cathode 5. A two-part anode 9 and 9' includes a conical cavity10, a radio frequency resonator cavity 11, and a second conical cavity12. The accelerating part 9 and the collector part 9' of the anode areelectrically insulated from each other by insulation 9". The cavities inthe anode 9 and the space surrounding the cathode 5 and the acceleratinggrid 8 are enclosed by glass ends 13 and 14 sealed to the acceleratinganode 9 and the collector anode 9', respectively. The space within thetube is evacuated to a pressure in the order of between 10* and 10millimeters of mercury.

The cathode 5 is maintained at a negative potential in the order of from250 to 1000 volts negative with respect to the accelerating grid 8 andthe anode 9, 9' by means of a battery 15.

A small resistor 16 is connected from the anode 9 to ground. Anode 9' isconnected directly to ground. The anode 9 is connected to the input ofan amplifier and phase adjuster 17. The output of the amplifier 17 isconnected to the accelerating grid 8, and thru a resistor 18 to ground.

Radio frequency energy, in the order of hundreds or thousands ofmegacycles from a source 29 is applied thru a coaxial line 21 to theresonant cavity 11 in the anode 9. The frequency of source 20 is thesame as the resonant frequency of the cavity. The radio frequency energyfrom source 20 which is applied to the tube reacts upon the electronbeam passing thru the resonant cavity 11 to provide an impedancetransformation so that the energy passes out of the glass seal 14 to aWaveguide (not shown).

The impedance transforming device is an example of one utilizationdevice which utilizes the concentrated electron beam. Other utilizationdevices such as klystrons, magnetrons, and traveling wave tubes may beemployed to receive the electron beam from the electron gun 5, 8, 9.

The amplifier and phase. adjuster 17 may be any suitable circuitsoperative at frequencies in the order of 0.5 to 10 megacycles, such asthe amplifier shown on page 435 and the phase adjuster shown on page 949of Termans Radio Engineers Handbook, first edition, 1943.

In operation, electrons are thermally emitted from the concave cathode 5and are attracted toward and thru the accelerating grid 8 and to theconical cavity portion 12 of the anode 9 because the grid 8 and anode 9,9 are 7 anode 9.

It is possible for the electron beam from cathode .5 to converge to forma very dense beam at the aperture 22 because the mutual repulsion of theelectrons in the beam is neutralized by the presence of a positive ionplasma in the confined region formed by the accelerating grid 8 and thewalls of the conical cavity 10 in anode 9. This region is substantiallyfree of any electric field that would tend to move the positive ionsfrom the region 'to an electrode surface. The positive ion plasma isformed 7 by electrons in the beam which strike residual gas molecules.

The positive ion plasma in the region between accelerating grid 8 andthe conical cavity 10 in anode 9 has a tendency to oscillate at afrequency determined by the density of the ion plasma. The oscillationsof the ion plasma'are usually caused by stray electrons which strike thewalls of cavity 10 and produce low energy secondary emission from thewalls of cavity 10 back into the ion plasma. The ion plasma oscillationcauses a modulation 7 of the electron beam passing thru the aperture 22in'the anode 9 V 'beam may be at a frequency somewhere between 0.5 andThis undesired modulation of the electron 10 'megacycles, and it has adisturbing effect upon the proper operation of the utilization device. f

The modulation of the electron beam by' the ion plasma is reflected inan oscillating partioning of the number of electrons striking the wallof the conical cavity 10 of the anode 9 compared with the numberStriking the wall of cavity 12 in anode 9'. This modulation istranslated to an alternating current voltage across resistor 16 which isconnected from the anode 9 to ground; The voltage across resistor 16 isapplied thru the amplifier and phase adjuster 17 to the accelerating'grid 8 in proper phase and magnitude so that the variation in thenumber of electrons V striking the walls of cavity 12 is substantiallyneutralized.

Inthe example of the invention shown and described, the

amplifier and phase adjuster 17 should preferably include an odd numberof vacuum tubes so that there is a ISO-degree phase reversal in theamplifier. At an instant of time when an excess number .of electronsstrike the Walls of conical cavity 10, the input to'the amplifier 17 isrelatively negative and an amplified positive signal is.

applied to the accelerating grid 8 to increase the number of'electronsin the beam. In this way, the modulation of the'electron beam and themodulation of the number of electrons reaching collecting anode .9 issubstantially V eliminated. A phase adjuster is desirable in the box 17so that adjustments may be made for minor phase shift in the completefeed-back loop.

It will be understood to those skilled'in the art that the neutralizingfeed-back signal from the amplifier 17 may be applied to the cathode 5rather-than .to the accelerating grid 8. The feed-back may be applied inany suitable manner so as to effect a neutralizing modulation 7 of thenumber of electrons entering the region formed oscillations from saidbeam, and -means to feed said V 4 signal voltage back to said source inphase opposition to substantially'cancel said modulation of the beam.

2. A vacuum tube system comprising, a cathode, a control electrode andan anode, means to bias said control electrode and said anode at apositive potential relative to saidcathode, said control electrode andsaid anode being formed to define a space charge neutralized region thruwhich an electron beam passes and in which ion plasma oscillations arecreated which modulate said beam, means to derive an alternating signalvoltage representa tive of said ion plasma oscillations from said anode,and means to feed said signal voltage across said cathode and controlelectrode in phase opposition to substantially cancel said modulation ofthe beam.

3. A vacuum tube system comprising a vacuum tube having electrodes togenerate an electron beam and having members forming a space chargeneutralized region thru which said beam passes and in which ion plasmaoscillations are created which modulate said beam, means to derive asignal voltage representative of said ion plasma oscillations from saidbeam, and means to feed said signal voltage back to said source in phaseopposition to substantially cancel said modulation of the beam.

4. A vacuum tube having .a first region wherein an electron beam isgenerated, a second space charge neutralized region thru which said beampasses and in which ion plasma oscillations are created which modulatesaid beam, and a third region wherein said beam is utilized, means toderive a signal voltage representative of said ion plasma oscillationsfrom said beam in said third region, and means to'feed said signalvoltage back to said' first region in phase opposition to substantiallycancel said modulationof thebeamin'said third region.

5. A-vacuum tube system comprising an evacuated envelope having aconcave cathode for producing a beam of electrons, a concentric grid,and an anode, said anode having a conical Wall at the apex of which theelectrons from said *cathode are concentrated, said grid and said anodeforming a space charge neutralized region therebetween in which ionplasma oscillations are created which modulate said beam, and feed-backmeans coupling energy r'epr'esentativeof said oscillations from saidanode to said grid. a

'6. A vacuumtube'system comprising an evacuated envelope having aconcave cathode for producing-a beam of electrons, 'a'concentric grid,an accelerating anode, and a collector anode, said accelerating anodehaving an aperture through which the electrons from said cathode pass tosaid collector anode, saidgrid and said. accelerating anode forming aspace charged neutralized region therebetween-in which ion plasmaoscillations are created which modulate said beam and cause a currentpartition between said accelerating anode and said collector anode, andfeed-back means coupling energy representative of said oscillations fromsaid accelerating anode to said grid.

7. A vacuum tube system as defined in claim 6, and in addition, radiofrequency utilization means coupled to said electron beam at a regionwithin said accelerating anode.

8. Avacuum tube'system as defined in claim 6, and in addition, aresonant cavity coupled to said electron beam and located between saidaccelerating anode and said collector anode, and a radiofrequency'source coupled to said resonant cavity.

References Citedin the file of thispatent UNITED STATES PATENTS 72,440,089 a Haetr r. 20, .1948 2,455,269 Pierce Nov. 30,1948 2,484,643Peterson. Oct. 11,1949 2,494,721 Robertson Jan. 17, 1950 2,595,698 PeterMay 6,1952 2,622,225 Linden Dec. 16, 1952 2,767,259 Petcr. Oct. 16,@1956

